A
view through a glass port into the green wood chip furnace.
Notice the blue flames.
This is a single chamber stratified combustor running with no secondary
air.
Chips: 50 lbs. of 47% moisture content green hemlock boughs,
including foliage. 2 hour burn.

During the winter of 2006 we heated our
Vermont home with wood chips. We tried both home grown chips and chips purchased from a sawmill. Our own chips were
cut from brush
growing on our 67 acre property -- a mix of black birch, red maple,
and cherry saplings, hemlock boughs and
assorted other species, burned green, straight from the chipper, with
an average moisture content of 43-47% (wet basis).

Not only did we burn green wood chips, but
our experimental furnace ran at high efficiency, and under normal
operating conditions, smoke-free. We would fill it with four
buckets of chips and in two hours producesan estimated
175,000 Btus of hot water in a heat storage tank to feed our
home baseboard heating system.

Our experimental furnace was located in a small
cinder block building which formerly housed a HAHSA II outdoor wood
furnace. (That furnace was removed due to pollution concerns and poor
performance.)

Thanks to a single chamber stratified batch
combustion method, we sometimes even burned chips mixed with snow and well
composted chips, again, cleanly and efficiently. The only
noticeable difference with these unlikely fuels is a partial reduction
in total
heat output.

Why use wood?

Wood
is still an abundant self-renewing material throughout the
United States. If it is harvested through managed forestry, woody waste
conversion, and active fuel wood cultivation it can remain a
self-sustaining resource and reduce our dependence on petroleum.

While
it's difficult to use wood for powering engines wood
can provide a boost in petroleum and gas supplies by replacing these
important fluid fuels in many heating applications. Simple
heating
does not require a sophisticated fuel like petroleum..

In
my small state of Vermont alone, it has been estimated that
low grade
timber could provide 1.5 million harvestable tons of green wood per year on a self renewing basis.
This is the yearly eqivalent of about 92 million gallons of home
heating oil which costs at
present prices, a quarter of a billion dollars.

“The
amount of wood available for whole-tree chip harvesting has been
conservatively estimated at one million green tons per year in
Northern Vermont alone.” Burlington
Vt. Electric Department

The
accelerating cost of fossil fuels now provides good reasons to
develop clean wood heat as a lower cost alternative to
petroleum. What
is needed is an available clean-burning
efficient method of converting wood into heat energy on a domestic
scale.

Why
wood chips?

There are three
kinds of common wood fuels: wood pellets, cord wood, and wood chips.

Pellets: Wood
pellets come from sawdust that is mixed with water,
extruded and dried with heat. They are then bagged in plastic, and
stored dry to be transported by truck and train from major
lumber producing
areas in the country.

Pellets burn
cleanly and efficiently. They are compact, and easy to handle. They are
a uniform product.

Unfortunately,
pellets are energy-intensive to make, store and transport, and
would be
unaffordable if they weren't made from lumber mill sawdust and
waste heat.This is becoming clear now that many people have
bought
pellet stoves.

Wood pellets
used to be priced competitively at $60 per ton a few years back.
However, last winter wood pellet prices reached $320 per
ton and there were shortages. This was because the
supply of cheap lumber mill sawdust has been depleted by the
demand
for pellets. On a heat-for-heat basis, the cost of pellets is now on a
par with petroleum
products as a fuel.

It's
hard to make more pellets, or lower their cost.
While sawdust can be ground entirely from logs, rather than gathered as
a byproduct of lumber mill operations, it is even more
costly to do this. Converting a whole tree to sawdust and
then gluing it back into pellets is not a particularly
efficient use of wood as an
energy resource.

Cord Wood: Cord
(log) wood has been burned since prehistoric people first made fires.
Everyone loves a log fire in a fireplace. Unfortunately a conventional
wood fire is not very efficient and
creates air pollution to a greater or lesser extent. Half of
the
gasified fuel products and heat of conventional wood fires is lost up
the
chimney.

With
the increased population in the United States, concerns
over pollution has forced the EPA to take action to limit the
adverse health effects of wood stove pollution by testing and
requiring emissions standards for manufacturers. This is making it more
difficult to provide affordable wood heat.

In response
manufacturers have
begun to use add-on catalytic converters or dual chamber gasification
methods to try to reclaim some of the heat, creosote, unburned carbon
monoxide
and pollutants that go up the chimney as smoke in a typical wood fire.
Unfortunately,
gasification furnaces can be very complex and expensive and there can
be
operational and maintenance issues due to creosote buildup in the
primary gasifying
chamber.

Legislation
has now been passed in some areas to ban outdoor wood
furnaces because of the high level of smoke pollution exhibited by the
worst of them. The
degree of pollution depends to a great extent on both design
and operator practice, but in general they produce higher
levels
of pollution and lower levels of efficiency than indoor wood stoves.

(The
VTHR experimental chip burning furnace is an exception to this rule,
but it was really an indoor type furnace housed in a dedicated masonry
shed rather than a standalone outdoor furnace.)

In other
pollution sensitive areas of the country, legislation against even
fireplaces has also
been
introduced.

Another
consideration with traditional wood burning is that generally
the most suitable, efficient fuel wood is good
hardwood from a tree from 6 to 18 inches in diameter. Such
trees
take up to a half century to grow and a large portion of the
tree -- the
limbs and branches -- is wasted when the tree is cut.

Burning
cord wood therefore presents some challenges for the future. In
addition, as a fuel, cord wood isn't necessarily easy to
use
or cost
free. Cord wood must be split, stacked and dried for a year to
burn efficiently in almost all wood stoves and furnaces. Dry
hardwood is particularly important for the expensive high-tech
dual-chamber gasifiers. Split dry hardwood prices are approaching
petroleum prices for equivalent heat.

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Wood Chips:
Wood
chips can be made from waste wood, brush, saplings, limbs, tree slash
from
logging
operations, and from forestry and roadside maintenance operations. Wood
chips do not need to be made from old standing timber. Other
advantages are:

Wood
chip prices are relatively stable between $15 and $30 per ton, and are
often available free from tree services working in the neighborhood..

The
energy required to chip wood is a fraction of the energy required to
make pellets. A wood chipper uses about the same amount of
energy per pound of product as a small wood splitter.

Green chips can be
used immediately without drying in stacks for a year (like
cord wood) and without using process energy (like pellets)..

Chips
can be transported and unloaded by dump truck. Because they are
generally available locally, long distance haulage, packaging, and
energy consumption can be reduced.

The supply of chips
is not limited by another industry's waste output.

Fuel
growing methods, such as brush and coppice farming can produce ideal
wood for chipping on a sustainable basis with a very high yield per
acre, and a short cycle. Coppicing can be mixed with conventional
timber forestry to maintain an ecological balance

Why don't we use chips to heat our homes?

One
reason wood chips haven't been used for home heating furnaces is that
there just hasn't been a long history of burning wood chips.
Conventional methods of small wood fire building obviously don't work
with wood chips.

This
complex and expensive
equipment is considered necessary because of the uneven
size of chipped wood – often mixed with twigs and sawdust.
This
mixture tends to jam in small feed mechanisms. You can't run chips through a pellet stove's 2"
auger.

The methods now
used to handle chips duplicates old petroleum fluid fuel handling
practices by
creating very large auger and belt “conduits”
compared to
the much smaller “fluid” chip particles to minimize
jamming.

The
usual burning method for wood chips also imitates fluid fuel practice
by using powerful blowers to circulate fuel particles in a big
combustion chamber.

As
a result of fluid fuel ideas, wood chips have been
used mainly for commercial and municipal scale operations, in schools,
and in power plants. Settings where the size
of the chip is small by comparison to the operational equipment.

Another
problem sometimes mentioned when discussing wood chip heat is the
mistaken idea that it is necessary to dry the fuel to a low
moisture content, as is done with pellets and cord wood. This is a
holdover of our prehistoric experience with wood fires, our
belief that a green log fire is nearly impossible to start or
maintain, and when burned emits lots of smoke, and little
heat.

Do everything wrong

My original aim in this
experiment was to explore a reversed approach to modern
commercial practices and conventional wisdom. Namely trying to directly
burn
wet green (50% moisture content) chips in a self-sustained reaction in
a small solidly packed single combustion chamber with little or no
secondary
air admission. I
wondered if doing this in a batch-burn high-output hot water
furnace would provide enough heat for a Vermont
home.

By
definition a batch process doesn't need continuous feed, or
throttling, and so eliminates large augers or combustion
chambers. As a result, the VTHR experimental furnace was about
the size and shape of a household hot water heater.

Because the furnace
uses a high output one-shot process, the
fire can't be modulated during the burn. The excess liberated heat
must be captured, stored, and later more slowly distributed to
the home where a thermostat can control the demand for
heat.

The experimental furnace stored heat in 2500 lbs of water in an insulated
tank. A small circulation pump deliverd this to the house. One
advantage of this kind of system is that other heat sources, such as
solar collectors, co generators, or even a conventional-fueled boiler
can be added in parallel, while baseboard heating units, radiators,
blown hot air exchangers, or domestic hot water tank exchanger can draw
heat from it.

This
allows a very flexible system. It also becomes easier to make
experimental changes in configuration, and monitor actual heat output,
losses, and usage.

How
do you burn wood well?

The problems
of burning wood are the result of 5 simple things.

Wood gives
off fuel gas when heated.

Gas needs to be
well mixed with air to burn well

Wood is thick
(massive).

Embers fall

Hot air rises

So
when we look at a fire in a fireplace on a grate what we really see is
some
unburned wood in the middle, hot embers heating that wood below, flames
and smoke rising above.

What's happening is:

The
hottest temperature in the fire is in the embers below and around the
relatively massive wood, heating it, and forcing it to emit cooler fuel
gases from within.

Air from the room
goes
to the embers first where it is depleted of oxygen.

The heated and
depleted air, mostly inert nitrogen, rises and mixes
with
the wood fuel gases without burning them.

Air also enters the
upper portion of the fireplace, but too little and too late
-- the pressure and
velocity of the hot gases is higher than the air pressure so mixing is poor, there's lots of nitrogen, and
temperature is actually dropping fast. The yellow
flames we call fire are actually cool flames with deficient
oxygen and lots of incandescent soot particles.

Smoke containing
fuel gases and creosote exiting above the flames is a result
of too little oxygen and too cool a temperature to support combustion
at all.